Manufacturing method of arc tube in excellent design, low-pressure mercury lamp, and illumination apparatus

ABSTRACT

An arc tube is composed of an arc tube body that is formed by winding a straight glass tube. Two portions of the straight glass tube are spirally wound around an imaginary conical surface. The spiraled portions of the glass tube are sandwiched between a pair of movable fixed plates. The spiraled portions of the glass tube are heated to a temperature equal to or higher than a point at which the glass tube is deformable and yet lower than the softening point of the glass tube. The spiraled portions are flattened by the movable plate moving downwardly under its own weight.

This application is based on an application No. 2003-394710 filed inJapan, the content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a manufacturing method of an arc tubecomposed of a glass tube having a spirally wound portion of which tubeaxis is contained within one plane. The present invention also relatesto the arc tube, a low-pressure mercury lamp, and an illuminationapparatus.

(2) Description of the Related Art

Low-pressure mercury lamps for general purpose illumination includesfluorescent lamps composed of a circular arc tube (hereinafter,“circular fluorescent lamp”). One feature of circular fluorescent lampslies in the flatness in shape.

Recently, there has been a growing demand for downsizing circularfluorescent lamps. This is because the downsizing of a circularfluorescent lamp leads to downsizing of an illumination apparatus foruse with the fluorescent lamp attached thereto.

One example of an arc tube that is smaller than conventional circulararc tubes is an arc tube composed of a glass tube that spirally windstoward an end thereof, around an imaginary axis farther and farther fromthe imaginary axis. The tube axis of the wound portion of the glass tubeis contained within one plane. This shape is referred to as a “flatspiral”, because the axis of the glass tube is contained within oneplane. (See JP Patent Application Publication No. 09-92154.)

With the glass tube having a flat spiral shape, the space enclosedwithin the circular arc tube is effectively utilized. Thus, the arc tubemay be made smaller in outermost diameter, without shortening thedischarge distance in comparison with that of the conventional arc tube.

The arc tube having a flat spiral shape is formed through a molding stepand a deforming step as follows. In the molding step, a softened glasstube is wound downwardly around the surface of a circular cone (thissurface is referred to as an “imaginary conical surface”), and thus theglass tube winds farther and farther away from the axis of the imaginaryconical surface (this axis is referred to as “imaginary axis”). Thisshape is referred to as a “stereo spiral”, because the axis of the glasstube shifts in the direction of the imaginary axis as the grass tubespirals. In the deforming step, the spirally wound glass tube is heatedthroughout its length, so that the glass tube deforms under its ownweight.

Unfortunately, however, there is a problem in the conventional method ofmanufacturing an arc tube into a flat spiral shape. According to theconventional method, upon flattening the wound portion of the glass tubein the direction of the imaginary axis, the glass tube is heated to itssoftening point in order to allow the glass tube to deform under its ownweight. As a result, the wound portion of the glass tube fails tomaintain its tubular shape. For this reason, it is often the case wherethe glass tube after the deforming is distorted in cross section,adjacent turns of the glass tube come into contact with each other, andthe end portion of the glass tube tapers. In short, according to theconventional method, it is difficult to manufacture an arc tube inexcellent design.

SUMMARY OF THE INVENTION

The present invention is made in view of the above problem and aims toprovide a manufacturing method of an arc tube having a flat spiral shapein excellent design. The present invention also aims to provide theabove arc tube, a low-pressure mercury lamp using the arc tube, and anillumination apparatus using the low-pressure mercury lamp.

To achieve the above aims, the present invention provides amanufacturing method of an arc tube, comprising: a winding step ofspirally winding a glass tube around an imaginary conical surface; and adeforming step of heating a spiraled portion of the glass tube to atemperature equal to or higher than a point at which the glass tube isdeformable and lower than a softening point of the glass tube, anddeforming the spiral so that the spiraled portion has a tube axissubstantially within one plane.

The glass tube may be spirally wound in part or throughout its length.The “part” used herein may refer to one or more portions of the glasstube. Furthermore, the spiraled portion of the glass tube may becompressed in part or thorough out its length. The “part” used hereinmay refer to one or more portions within the spiraled portion of theglass tube.

Note that the term “imaginary conical surface” used herein assumes thelateral surface(s) of a circular cone as well as of a pyramidal cone.The circular cone includes a circular cone, an elliptical cone, and soon. The pyramidal cone includes a triangular pyramid, a quadrangularpyramid, and soon. Note, in addition, that the term “deformable” refersto the state where the glass tube is plastic deformable.

According to the manufacturing method stated above, in the compressingstep, the spiraled portion of the glass tube is heated to a temperaturethat is equal to or higher than a point at which the glass tube becomesdeformable and yet lower than the softening point of the glass tube.Consequently, the glass tube is prevented from being distorted in crosssection, making contact between adjacent turns, or being tapered at theend portion. Thus, the arc tube having a flat spiral shape ismanufactured in excellent design.

Here, when seen from an axis of the imaginary conical surface, a minimumgap between adjacent turns of the spiraled portion before the deformingis not be smaller than 0.5 mm and not greater than 0.4 times an outsidediameter D1 of the glass tube.

Note that the term “axis of the imaginary conical surface” refers to anaxis of a circular or pyramidal cone defining the imaginary conicalsurface.

In another aspect, the present invention provides an arc tube comprisinga glass tube having at least one portion spirally wound around animaginary axis. The spiraled portion of the glass tube has a tube axissubstantially within one plane perpendicular to the imaginary axis. Aminimum gap between adjacent turns of the spiraled portion is not lessthan 0.5 mm and not greater than 0.4 times an outside diameter D1 of theglass tube in mm.

BRIEF DESCRIPTION OF THE DRAWINGS

These and the other objects, advantages and features of the inventionwill become apparent from the following description thereof taken inconjunction with the accompanying drawings which illustrate a specificembodiment of the invention.

In the drawings:

FIG. 1 is a front view showing a fluorescent lamp according to apreferred embodiment of the present invention, seen from itsillumination side;

FIG. 2 is a partly-broken view of the fluorescent lamp shown in FIG. 1,seen from the direction of an arrow X to show the inside of a holder andan arc tube;

FIG. 3 is a disassembled, partly-broken oblique view of the holder shownin FIG. 2;

FIG. 4 is a partly-broken side view showing an illumination apparatushaving the fluorescent lamp shown in FIG. 1;

FIG. 5 is a view illustrating steps of manufacturing an arc tube bodyaccording to the preferred embodiment;

FIG. 6A is a partly broken view of an intermediate product shown in FIG.5(b);

FIG. 6B is a front view of the intermediate product seen from thedirection of an arrow Y shown in FIG. 6A

FIG. 7 is a side view of a molding jig;

FIG. 8 is an enlarged cross-sectional view of grooves;

FIG. 9 is a view illustrating a molding step of the intermediateproduct;

FIG. 10 is a view illustrating a step of compressing the intermediateproduct to a substantially flat shape;

FIG. 11 is a partly-broken oblique view of the compressing jig alongwith the intermediate product;

FIG. 12 is a table showing subjective evaluations of unevenness in thebrightness of fluorescent lamps;

FIG. 13 is a plan view of a fixed plate of the compressing jig forforming an arc tube body having a substantially uniform gap between anytwo adjacent turns;

FIG. 14A is a view showing a modified shape of the intermediate product;

FIG. 14B is a view showing a modified shape of the arc tube body; and

FIG. 15 is a view showing an arc tube body formed by flattening part ofwound portions of the intermediate product.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following describes a fluorescent lamp 10, which is a type of alow-pressure mercury lamp, as a preferred embodiment of the presentinvention.

I. Structure of Fluorescent Lamp

FIG. 1 is a front view showing the fluorescent lamp 10 of the preferredembodiment, seen from its illumination side. FIG. 2 is a partly-brokenview of the fluorescent lamp 10 seen from the direction of an arrow X toshow the inside of a holder 200 and an arc tube 100.

As shown in FIGS. 1 and 2, the fluorescent lamp 10 is generally composedof the arc tube 100 through which one discharge path is formed, and theholder 200 holding the arc tube 100. For electronic power supply, alater-described base 250 is mounted on the holder 200.

(1) Arc Tube

As shown in FIGS. 1 and 2, the arc tube 100 is composed of an arc tubebody 110 and a pair of electrodes 140. The arc tube body 110 is formedof, for example, a straight glass tube 112 that is wound into a spiralform. The electrodes 140 are provided within the arc tube body 110 athermetically-sealed ends 114 and 116. The arc tube body 110 is filledwith mercury (in an amount of 5 [mg], for example), in addition to anargon gas serving as a buffer gas (at a pressure of 400 [Pa], forexample).

Note that FIG. 2 does not show the electrode 140 that is provided at theend 116 of the arc tube body 110, for the convenience sake of theillustration. Each electrode 140 has an identical structure.

In addition, the mercury enclosed within the arc tube body 110 may be asimple substance or a mercury amalgam with zinc, tin, or bismuth-indium.

As shown in FIGS. 1 and 2, the glass tube 112 has a middle portion 120and two wound portions 122 and 124. The wound portions oppositely extendfrom the middle portion 120 and spiral around an imaginary axis A, whichwill be described later.

The wound portions 122 and 124 of the glass tube 112 have the tube axiscontained substantially within one plane that is substantiallyperpendicular to the imaginary axis A. The wound portions 122 and 124wind farther and farther from the imaginary axis A toward the respectiveends 114 and 116. In other words, the wound portions 122 and 124 form aflat spiral wound around the imaginary axis A.

As shown in FIGS. 1 and 2, the middle portion 120 has a bulge 126 at aposition corresponding to the imaginary axis A of the spirally wound arctube body 110, i.e. at the center of the middle portion 120. The bulge126 protrudes in one direction of the imaginary axis A and serves as thecold spot during the time the florescent lamp 10 is operated. The coldspot is a point on the arc tube 100 where the temperature is lowest.This lowest temperature determines the pressure of mercury vapor withinthe arc tube 100 under the lamp operation.

The glass tube 112 may be made of barium-strontium silicate glass(lead-free glass) and substantially circular in cross section. Yet, thecross-sectional shape of the glass tube 112 is not limited to a circularshape. Alternatively, the glass tube 112 may be substantially ellipticalin cross section. In any case, the arc tube body 110 has more or less anirregular cross-sectional shape, rather than a perfect circle. This isbecause the arc tube body 110 is formed into a flat spiral shape bysoftening and winding the straight glass tube 112.

As shown in FIG. 1, the arc tube body 110 when seen from theillumination side has a gap between each two adjacent turns of woundportions 122 and 124 in a direction perpendicular to the imaginary axisA (hereinafter, this perpendicular direction is referred to as a “radialdirection”).

Since the glass tube 112 is substantially circular in cross-section, thegap between each two adjacent turns of the wound portions 122 and 124 issmallest on the plane containing the tube axis of the wound portions ofthe glass tube 112. This smallest gap is denoted as a gap Ga.

At the ends 114 and 116 as well as their neighboring portions of the arctube body 110, the gap between adjacent turns of the wound portions 122and 124 is larger. The gap is made larger in order to prevent the nearbyportions from being deformed when the ends 114 and 116 are heated to besealed with the electrodes 140. Thus, the gap Ga is formed between anytwo adjacent turns of the wound portions 122 and 124, excluding the ends114 and 116 and their neighboring portions. (The neighboring portionsmay be portions inwardly extending from the ends 114 and 116 to a pointforming a radial angle of about 45° with the respective ends.)

As shown in FIG. 1, the inner surface of the arc tube body 110 iscovered with a phosphor 160. The phosphor 160 is composed of, forexample, red, green, and blue rare-earth phosphors, such as Y₂O₃:Eu,LaPO₄:Ce, Tb and BaMg₂Al₁₆O₂₇:Eu, and Mn.

Each electrode 140 is so-called “glass-bead mounting” type, and iscomposed of a tungsten electrode coil 142, a pair of lead wires 146 and148 supporting the electrode coil 142 thereacross, and a glass bead 144fixedly supporting the lead wires 146 and 148.

The pair of electrodes 140 is individually attached to the respectivesealed ends 114 and 116 of the arc tube body 110 by partially embeddingthe lead wires 146 and 148 therein. Specifically, part of the lead wires146 and 148 extending from the glass bead 144 oppositely to theelectrode coil 142 is embedded.

To one end of the arc tube body 110 (the end 114, in this embodiment),an exhaust pipe 150 is attached by sealing the end along with theelectrode 140. The exhaust pipe 150 is used, after the electrode 140 isattached, for evacuating the arc tube body 110 or introducing a buffergas into the arc tube body 110.

To sum up, the arc tube 100 is composed of the arc tube body 110 having:the phosphor 160 applied over its inner surface; the electrodes 140attached by sealing the respective ends thereof; and the buffer gasenclosed therein. The ends 114 and 116 and the middle portion 120 arethe terms referring to the portions of the arc tube body 110.Hereinafter, the same terms and numerals are used to refer to thecorresponding portions of the arc tube 100. Similarly, the term “radialdirection” of the arc tube body 110 is also used to refer to the radialdirection of the arc tube 100.

Note that the arc tube 100 is attached to the holder 200 in a mannerthat the side having the bulge 126 faces away from the holder 200 (i.e.the bulge 126 is exposed at the illumination side).

(2) Holder

FIG. 3 is a disassembled, partly-broken oblique view showing the holder200 according to the present embodiment.

As shown in FIGS. 1-3, the holder 200 is composed of a holding member210 and a base-mounting member 230 for the mounting of the base 250. Thebase 250 has four connecting pins 250 a, 250 b, 250 c, and 250 d forconnection with electronic power supply.

The holding member 210 is composed of a main body 212 of whichlongitudinal direction extends across the ends 114 and 116 of the arctube 100. The main body 212 has projections 214 and 216 each at one oflongitudinally opposing ends. The projections 214 and 216 have insertionholes 218 and 220 for inserting the ends 114 and 116 of the arc tube100.

The insertion holes 218 and 220 each have an L shape composed of a firsthole and a second hole. The first hole extends laterally of theprojection 214 (216), and has a shape conforming to the profile of theend 114 (116) of the arc tube 100. The second portion bends from thefirst hole and extends toward the rear side of the main body 212 (i.e.extends upwardly).

With the above structure, the arc tube 100 is securely supported as aresult of the surface engagement of the ends 114 and 116 against theinner surfaces 220 a of the first hole (the inner surface of theinsertion hole 218 is not illustrated in the figure). In addition, thepairs of lead wires 146 and 148 each extending from the ends 114 and 116are directed to the rear side of the main body 212 through the upwardlyextending second hole (See FIG. 2).

As shown in FIG. 2, the holding member 210 has a space 222 securedinside thereof. The space 222 allows the pairs of lead wires 146 and 148each extending from the ends 114 and 116 of the arc tube 100 to beguided to the base 250. The base-mounting member 230 is fitted withinthe holding member 210 so as to close the space 222 from above. Thebase-mounting member 230 is fixed to the holding member 210 for exampleby an adhesive.

(3) Illumination Apparatus

FIG. 4 is a partly-broken side view showing an illumination apparatus400 having the fluorescent lamp 10 of this embodiment.

As shown in the figure, the illumination apparatus 400 may be aceiling-mounted type. A main body 410 of the illumination apparatus 400is attached, for example, via a socket 440 to a rosette provided withina ceiling 430.

The main body 410 is composed of a shade 414 having a flat bottom 412substantially at the center thereof. The fluorescent lamp 10 isremovably attached inside the shade 414 onto the bottom 412. The socket440 is provided on the outer surface of the bottom 412 of the shade 414.The socket 440 houses therein an electronic ballast (not illustrated)for lighting the fluorescent lamp 10.

The fluorescent lamp 10 is removably attached to the main body 410 byinserting the base 250 (see FIG. 2, for example) to another socket (notillustrated) provided inside the shade 414. Upon attachment, thefluorescent lamp 10 is brought into electrical connection. The electricballast is of a series-inverter type dedicated for high-frequency.

The inner surface of the shade 414 may be a reflecting surface thatreflects light emitted from the fluorescence lamp 10 to illuminate adesired direction, for example, a downward direction. The reflectingsurface may be formed by applying a white paint or alumina particlesover the inner surface of the shade 414.

When the electric ballast turns on the fluorescence lamp 10, the coldspot is formed at the bulge 126 of the arc tube 100. The arc tube 100 isdesigned such that the temperature at the cold spot (so-called cold spottemperature) under the steady state operation reaches a value thatoptimizes the pressure of mercury vapor for achieving the maximum lampefficiency. The pressure of mercury vapor is regulated based on the coldspot temperature. This is because, as described above, the cold spottemperature uniquely determines the pressure of the mercury vapor underthe steady state operation.

II. Specific Structure of Fluorescent Lamp

The fluorescent lamp 10 according to the present invention is providedin an attempt to downsize a conventional circular fluorescent lamp.First, a description is given to a conventional circular fluorescentlamp comparable to the fluorescent lamp 10. The conventional circularfluorescent lamp mentioned herein is a so-called “Circular 20” slimtype, and is made of a glass tube of which outside diameter is 16 [mm].The glass tube has a circular shape of which outside and insidediameters of the circle are 225 [mm] 192 [mm], respectively. Inaddition, the total luminous flux is 2310 [mm] and the rated operatinglife is 9000 [hr].

Now, a description is given to the specific structure of the fluorescentlamp 10. An outside diameter D1 of the glass tube 112 is 9.0 [mm] and aninside diameter D2 is 7.4 [mm] (See FIG. 1).

As stated above, the two wound portions 122 and 124 of the arc tube body110 are spirally wound around the imaginary A, and the number of turnsof the spiral is about 4.0 full turns in total. As shown in FIG. 1, thedimensions of the arc tube 100 are such that a length L1 connecting theends 114 and 116 is 120 [mm], and a length L2 of the diametric distanceperpendicular to the line segment connecting the ends 114 and 116 is 110[mm]. The gap Ga between radially adjacent turns of the glass tube 112is about 1.0 [mm].

That is to say, the outside diameter of the fluorescent lamp 10 is 0.53times the outside diameter of the conventional circular fluorescentlamp. The height of the fluorescent lamp 10 including the bulge 126 is11 [mm], which is 0.69 times the height of the conventional circularfluorescent lamp. The height of the fluorescent lamp 10 excluding thebulge 126 is equal to the outside diameter of the glass tube 112, whichis 9 [mm] and is 0.56 times the height of the conventional fluorescentlamp.

The interelectrode distance of the above arc tube 100 is 700 [mm]. Whenthe fluorescent lamp 10 composed of the arc tube 100 was turned on in abase-up position with a lamp input of 27 [W], the resulting luminanceflux was 2220 [lm] and lamp efficiency was 82.2 [lm/W].

The luminous flux emitted by the fluorescent lamp 10 is slightly lowerthan the luminous flux of 2310 [mm] emitted by the conventional circularfluorescent lamp. Yet, the difference is negligible to regard themsubstantially equal.

In the lamp operation mentioned above, the bulb wall loading imposed onthe arc tube 100 was 0.17 W/cm² and the rated operating life was 11,000[hr]. The “rated operating life” used herein is determined based ontesting. In the testing, a fluorescent lamp having the arc tube 100 wasrepeatedly turned on for 2.75 hours and turned off for 0.25 hours. Thetime at which the lamp no longer emit light was measured, as well as thetime at which the total luminous flux was reduced to 60% or less of thetotal luminous flux measured after 100 hours of operation. The ratedoperating life hours were the shorter one of the time periods from theoperation start to the two points of time measured.

Note that the rated operating life of the arc tube 100 is about 1.2times longer than that of the conventional circular fluorescent lamp,which is 9000 [hr].

III. Manufacturing Method of Arc Tube

Now, a description is given to a manufacturing method of the arc tube100 having the above structure.

FIG. 5 is a view illustrating steps of manufacturing an arc tube body.

The manufacturing method described herein is for manufacturing the arctube body 110 described above. First, the flow of manufacturing steps isexplained briefly, and then each step is explained in detail.

First of all, as shown in (a) of FIG. 5, a straight glass tube 510 isprepared. The glass tube 510 is heated to be softened, and then spirallywound around an imaginary conical surface of a molding jig 590, whichwill be described later. As a result, the glass tube 510 is formed intoan intermediate product 540 having a spirally wound portion. As shown in(b) of FIG. 5, the intermediate product 540 is substantially conical inoutline, when seen from a direction perpendicular to the axis of themolding jig.

In the next step, the intermediate product 540 is heated again, andcompressed in the axial direction of the imaginary conical surface toflatten the wound portion. As a result, the arc tube body 110 having aflat spiral shape is formed.

Note that the term “flat” used herein refers to a shape of that thethickness of the arc tube body is substantially equal to the outsidediameter of the glass tube, when seen from a direction perpendicular tothe axis of the molding jig at the time of the molding.

Thereafter, the arc tube body 110 is subjected to a step of applying aphosphor coating to the inner surface of the tube, a sealing step ofattaching the electrodes by sealing each end of the tube, and a gasenclosing step of enclosing mercury and an argon gas inside the arc tubebody 110. Yet, these steps are carried out according to conventionaltechniques. Thus, the description thereof is omitted.

Hereinafter, a description is given to a molding step of a conicalintermediate product (equivalent to the winding step of the presentinvention) and to a compressing step of the intermediate product flat(equivalent to the deforming step of the present invention).

(1) Molding Step of Intermediate Product

A. Glass Tube

First of all, a description is given to the glass tube 510. Before thewinding, the glass tube 510 is straight and has three portions 510 a,510 b, and 510 c in the longitudinal direction, as shown in (a) of FIG.5. The portions 510 b and 510 c are portions to be subjected to thewinding. The portion 510 a is the remaining portion between the twoportions 510 b and 510 c, and referred to as a middle portion. Whenprocessing the glass tube 510 into the intermediate product 540, themiddle portion 510 a and at least part of the portions 510 b and 510 care softened by heat. The glass tube 510 is substantially circular incross section, 9.0 [mm] in outside diameter, 7.4 [mm] in insidediameter, and 1500 [mm] in entire length.

B. Intermediate Product

The intermediate product 540 is formed by winding the middle portion 510a and the portions 510 b and 510 c of the glass tube 510.

FIG. 6A is a partly broken view of the intermediate product 540 shown in(b) of FIG. 5. FIG. 6B is a front view of the intermediate product 540,seen from the direction of an arrow Y shown in FIG. 6A.

As shown in FIGS. 6A and 6B, the intermediate product 540 has two woundportions 548 and 550 and a middle portion 542 connecting the two woundportions 548 and 550. The wound portions 548 and 550 are formed bywinding the portions 510 b and 510 c of the glass tube 510, and themiddle portion 542 corresponds to the middle portion 510 a of the glasstube 510.

The wound portions 548 and 550 are formed by winding the portions 510 band 510 c of the glass tube 510 downwardly toward the respective ends,around the imaginary conical surface of the molding jig. Thus, the woundportions 548 and 550 spiral farther and farter from the middle portion510 a in the direction of the imaginary axis (the downward direction inFIG. 6A), and from the imaginary axis in the radial direction. (Notethat during the winding of the glass tube, the imaginary axis coincideswith the axis of the molding jig having an imaginary conical surface.The imaginary axis is equivalent to the imaginary axis of the conicalsurface of the present invention.) Thus, the intermediate product 540has a substantially conical shape in its outline. The generating lineand the axis of conical shape form an angle of 60°, which is denoted as“α” in FIG. 6A.

The positional relation between the wound portions 548 and 550 is suchthat a gap is formed between each two radially adjacent turns of thewound portions 548 and 550, when seen from the extending direction ofthe imaginary axis A (i.e. the direction of the arrow Y shown in FIG.6A). FIG. 6B shows the intermediate product 540 from the direction ofthe arrow Y. The minimum length L3 of the gap is 1.0 [mm].

Further, when seeing the intermediate product 540 from the directionperpendicular to the imaginary axis A (as shown in FIG. 6A), each turnof the wound portions 548 and 550 partially overlap an immediatelyinside turn in the direction of the imaginary axis A. The length L4 ofthe height of overlapping portion is 4.0 [mm]. The number of turns ofthe wound portions 548 and 550 is four in total.

C. Jig for Forming Intermediate Product

The molding jig 590 has the imaginary conical surface, and is providedwith a pair of latching members 593 and 594. The intermediate product540 is formed by placing the middle portion 510 a of the softened glasstube 510 between the pair of latching members 593 and 594, and thenwinding the portions 510 b and 510 c.

FIG. 7 is a side view of the molding jig 590.

As shown in the figure, the molding jig 590 is composed of a main body591 and a columnar mounting portion 592 for mounding the molding jig 590to a driving device (not illustrated). The softened glass tube 510 iswound around the outer surface of the main body 591. The axes of themain body 591 and the mounting portion 592 coincide. In FIG. 7, the twoaxes are collectively illustrated as the axis of the molding jig 590 anddenoted by the reference sign “B”. In the following description, adirection perpendicular to the axis B is referred to as the “radialdirection”.

The latching members 593 and 594 are provided upright at the top of themain body 591 in diametrically opposed positions. In addition, twogrooves 595 and 596 are formed along the main body 591 from the top tothe bottom in a manner defining one continuous spiral. The grooves 595and 596 receive therein the portions 510 b and 510 c of the glass tube510.

The latching members 593 and 594 are spaced by an amount to allow theglass tube 510 to be fit therebetween. Each of the latching members 593and 594 extends out from the top of the molding jig 590 in a directionparallel to the axis B. The latching members 593 and 594 may be a pairof rod-like members, such as pins, mounted to the molding jig 590 inparallel to the axis B.

The rod-like members used in this embodiment are circular incross-section. Yet, the rod-like members may have any other shape aslong as the surfaces to contact with the glass tube 510 are circular. Inaddition, the rod-like members may be tapered or gradually thinnertoward the end (upper end).

That is to say, the latching members 593 and 594 may be in any shapeensuring the glass tube 510 not to be snagged at the time of disengagingthe glass tube 510 from the molding jig 590 in the axial direction (forexample, by moving the molding jig 590 downward).

FIG. 8 is an enlarged cross-sectional view of the grooves 595 and 596.

As shown in FIG. 8, the grooves 595 and 596 are formed in the outersurface of the main body 591 and a staircase shape. In cross section,the grooves 595 and 596 each define a rounded corner 597 of whichcurvature is equal to that of the outer circumference of the glass tube510 (shown in FIG. 8 in phantom lines).

With the above structure, the glass tube 510 is brought into surfacecontact with the round corner 597 at part as follows. First, thecross-sectional face of the glass tube 510 is bounded by two diametricallines, one parallel to, and the other perpendicular to the axis B. Thecontacting surface is the lower quadrant of the outer circumferentialsurface that is closer to the axis B (the line segments C-E in FIG. 8).This structure prevents that the glass tube 510 wound around the moldingjig 590 easily comes off from the grooves 595 and 596.

Here, since the glass tube 510 wound around the molding jig 590 has asubstantially conical shape, the outer surface of the main body 591substantially defines a conical shape and thus considered as theimaginary conical surface (equivalent to the imaginary conical surfaceof the present invention).

Next, a description is given to the specific structure of the moldingjig 590. The molding jig 590 described herein is for manufacturing thearc tube body 110 described in the above section of “2. SpecificStructure of Fluorescent Lamp”. The height H (see FIG. 7) of each of thegrooves 595 and 596 is about 0.56 times the outside diameter D1 of theglass tube 510. The reason for determining the height H as such isdescribed later.

The inventors of the present invention have found the following, basedon testing. When the height H is at least 0.5 times the outside diameterof the glass tube 510, the winding is carried out without such anoccurrence that the glass tube 510 comes off from the grooves 595 and596 of the molding jig 590. Thus, the winding is carried out with highmanufacturing yield.

Furthermore, the width W (i.e. overhang) of each of the grooves 595 and596 is larger by about 1.0 [mm] than the outside diameter D1 of theglass tube 510. The reason for determining the width W as such isdescribed later.

An angle β formed between the generating line B1 and the axis B both ofthe imaginary conical surface is about 60°.

To sum up the above, the pitch of the grooves 595 and 596 in the axialdirection is equal to a value calculated by subtracting L4 from D1,where D1 is the outside diameter of the glass tube 510, and L4 is theheight of the overlapping portion of adjacent turns of the woundportions 548 and 550 (see FIG. 6A). Further, the pitch of the grooves595 and 596 in the radial direction is equal to a sum of D1 and L3,where D1 is the outside diameter of the glass tube 510, and L3 is thewidth of the gap between adjacent turns of the wound portions 548 and550.

D. Forming Step of Intermediate Product

FIG. 9 is a view illustrating the molding step for obtaining theintermediate product 540.

As shown in the figure, the mounting portion 592 of the molding jig 590is attached to the driving apparatus, which is not shown in the figure.The driving apparatus is capable of moving the molding jig 590 in thedirection of G shown in (a) of FIG. 9, while rotating the molding jig590 on the axis B in the direction of F also shown in the figure.

Next, the glass tube 510 is heated at least partially (at least part ofeach of the portions 510 a, 510 b, and 510 c), for example, in a furnaceto a temperature of 800° C.±20° C. As a result, the heated part of theglass tube 510 is softened.

The middle portion 510 a of the softened glass tube 510 is generallycentrally inserted between the pair of latching members 593 and 594 ofthe molding jig 590. As shown in (a) of FIG. 9, each end of the glasstube 510 is grasped in place. Under this state, the molding jig 590 isrotated on the axis B in the direction of F and concurrently moved inthe direction of G.

As a result, the middle portion 510 a of the glass tube 510 is fixed bythe latching members 593 and 594, and the two portions 510 b and 510 care wound along the grooves 595 and 596 (i.e. around the imaginaryconical surface).

Under this state, the grooves 595 and 596 receive the glass tube 510 andthus in surface contact with the outer circumference of the glass tube510 at the point Con the receiving surface, as shown in phantom lines inFIG. 8. The contact prevents detachment of the glass tube 510 from thegrooves 595 and 596.

Note that the movement amount of the molding jig 590 in the direction ofG per rotation is equal to the total height of the grooves 595 and 596,i.e. twice the height H. During the rotation, a gas such as nitrogen orargon is introduced into the glass tube 510 at a controlled pressure. Asa result, the wound portion of the grass tube 510 comes to have acircular cross section.

After completing the winding around the molding jig 590, the glass tube510 is left to stand until the temperature lowers and the glass tube 510hardens. Then, the hardened glass tube, which is denoted by a referencenumeral 515, is removed from the molding jig 590 in the axial direction.

To be more specific, as shown in (b) of FIG. 9, the molding jig 590 maybe axially moved downward in the direction of I while the glass tube 515is kept in place. Alternatively, the glass tube 515 may be moved in thereverse direction of the direction I (in the G direction) while themolding jig 590 is kept in place. Alternatively, both the glass tubes515 and the molding jig 590 may be moved.

Note that when forming a plurality of glass tubes 510 one after another,the molding jig 590 needs to be brought back to its initial position.Thus, it is advantageous in terms of manufacturing efficiency to movethe molding jig 590 downward in the direction I.

After removal from the molding jig 590, the glass tube 515 is cut offits unnecessary portion, thereby the glass tube 515 is formed into theintermediate product 540.

At the apex of the intermediate product 540, a protrusion 552 is formed.The protrusion 552 later serves as the bulge 126 of the arc tube 100.The protrusion 552 is formed by locally softening the apex of theintermediate product 540 and increasing the internal pressure of theintermediate product 540. The protrusion 552 maybe formed immediatelyafter winding the glass tube around the molding jig 590 or afterremoving the glass tube from the molding jig 590.

(2) Compressing Step of Intermediate Product

Next, a description is given to the step of compressing thesubstantially conical intermediate product to flatten, thereby obtaininga complete arc tube body.

FIG. 10 is a view illustrating the step of compressing the intermediateproduct to a substantially flat shape.

This step is carried out using a compressing jig 580 as shown in (a) ofFIG. 10. Thus, a description is given first to the compressing jig 580,and then to the compressing step.

A. Compressing Jig

FIG. 11 is a partly-broken oblique view of the compressing jig 580 onwhich the intermediate product is placed.

As shown in FIGS. 10 and 11, the compressing jig 580 has such astructure for sandwiching the intermediate product 540 in the directionof the imaginary axis A (the up and down direction in FIG. 11). To thisend, the compressing jig 580 is provided with a pair of upper and lowerplates and a plurality of guide pins 586. The pair of plates move alongthe guide pins 586 closer to or farther from each other, whilemaintaining their opposing surfaces in spaced parallel relation.Specifically, the lower plate is a fixed plate 582 having a placementsurface 582 a for placing the intermediate product 540 thereon. Theupper plate is a movable plate 584 that is movable along the guide pins586 in the direction perpendicular to the placement surface 582 a of thefixed plate 582 (in the up and down direction in FIG. 11).

The plurality of guide pins 586 is, for example, six guide pins that areprovided upright on the fixed plate 582 in a circumferentially spacedrelation along the outermost circumference of the intermediate product540. In addition, stoppers 589 are provided on the fixed plate 582 alongthe periphery of the placement surface 582 a (In the example shown inFIG. 11, the placement surface 582 a is rectangular, so that thestoppers 589 are provided each at one of the four corners). By thepresence of the stoppers 589, the movable prate 584 is regulated fromcoming too close to the fixed plate 582.

The movable plate 584 has a through hole 587 generally at a centerthereof, for accommodating the protrusion 552. The movable plate 584also has six guide holes 585 at positions corresponding to the guidepins 586.

Now, a description is given to the specific structure of the compressingjig 580.

The fixed plate 582 and movable plate 584 are each made of stainlesssteel and substantially square in plan view. The movable plate 584weighs about 1.2 [Kg].

The guide pins 586 have an outside diameter of 2.5 [mm] and provided onthe fixed plate 582 at circumferential intervals of 35 [mm]. Thediameter of each guide hole 585 is 2.6 [mm].

B. Compressing Step

Now, a description is given to the compressing step of flattening theintermediate product 540.

First of all, the compressing jig 580 is prepared, and the intermediateproduct 540 is placed between the plates 582 and 584, as shown in (a) ofFIG. 10. For easy placement of the intermediate product 540, the movableplate 584 may be detached or moved upward to secure a wide space betweenthe plates 582 and 584.

The intermediate product 540 is placed in a predetermined position onthe placement surface 582 a of the fixed plate 582. For example, theintermediate product 540 is placed generally at the center of theplacement surface 582 a. When the intermediate product 540 is properlyplaced, the protrusion 552 of the intermediate product 540 comes toenter the thorough hole 587 upon downward movement of the movable plate584. At this time, the peripheral wall of the through hole 587 engagesagainst the middle portion 542 of the intermediate product 540.

Next, while maintaining the engagement of the movable plate 584 againstthe middle portion 542, the intermediate product 540 is heated as shownin (b) of FIG. 10, until the outer peripheral surface of the glass tubereaches about 600° C. The reason for determining the target temperatureas such will be described later.

Note that the intermediate product 540 is made of glass (soft glass) ofwhich softening point is 675° C. When reaching temperatures equal to orhigher than the softening point, the intermediate product 540 starts todeform under its own weight and thus becomes flattened. Upon thedeforming, the glass tube tends to undergo distortion in its crosssectional shape.

When the temperature of the outer circumferential surface of the glasstube reaches about 600° C., the intermediate product 540 becomesdeformable by force. As a consequence, the movable plate 584 starts todescend (move toward the fixed plate 582) under its own weight, therebycompressing and deforming the intermediate product 540 in the directionof the imaginary axis. The intermediate product 540 is continuallydeformed until the movable plate 584 engages against the stoppers 589 ofthe fixed plate 582.

When the movable plate 584 comes to engage against the stoppers 589, theintermediate product 540 is flattened except for the protrusion 552accommodated in the through hole 587, as shown in (c) of FIG. 10. As aresult, the arc tube body 110 having flat wound portions 122 and 124 isobtained.

The arc tube body 110 manufactured according to the above manufacturingmethod is deformed flat by force at a temperature lower than thesoftening point of the glass tube 510. That is to say, throughout thedeforming process, the glass tube retains its rigidity to an extent ofmaintaining its tubular shape. As a result, unlike the conventionaltechnique, the conical intermediate product 540 is deformed flat withoutsuch problems as distortion of the glass tube in cross section or axialprolongation of the glass tube.

Furthermore, the compressing jig used for deforming the intermediateproduct 540 exploits the weight of the movable plate 584 and thus isrelatively simple in structure. This simple structure of the jig helpsto reduce the manufacturing cost of the arc tube body 110.

Note that the intermediate product 540 is deformed in the direction ofthe imaginary axis without causing distortion of the glass tube in crosssection. This is because the force required to distort the glass tube incross section is greater than the force recurred to deform (compress)the intermediate product 540 in the direction of the imaginary axis.

IV. Assembling Fluorescent Lamp

Next, a description is given to assembling the arc tube 100 manufacturedin the above-described manner and the holder 200 into a fluorescentlamp.

First, the arc tube 100 and the holder 200 are prepared. The arc tube100 is attached to the holder 200 by inserting the ends 114 and 116 intothe respective insertion holes 218 and 220 formed through the holdingmember 210. Then, the ends 114 and 116 are secured within the insertionholes 218 and 220 by adhesive such as silicon resin. The holder 200 atthis stage is not yet attached to the base-mounting member 230.

Next, the lead wires 146 and 148 extending from the ends 114 and 116 ofthe arc tube 100 are routed thorough the connecting pins 250 a, 250 b,250 c, and 250 d of the base 250 that is attached to the base-mountingmember 230. The lead wires 146 and 148 other than the part routedthrough the pins are placed into the space 222. Then, the base-mountingmember 230 is attached on the rear side of the holding member 210, andthe connecting pins 250 a, 250 b, 250 c, and 250 d are swaged. Thiscompletes the fluorescent lamp 10.

V. Gap between Adjacent Turns of Glass Tube

The arc tube 100 described above is designed so that the minimum gap Gabetween radially adjacent turns is 1 [mm]. However, the gap Ga may beany size not exceeding 0.4 times the outside diameter D1 of the glasstube 112. The reason for determining the minimum gap Ga as such isdescribed below.

The fluorescent lamp 10 according to the present invention is expectedto be applied to a thin ceiling-mounted type illumination apparatus forstore lighting as well as home lighting. Especially, the fluorescentlamp 10 is expected to be applied more often to illumination apparatusesof an open type than a closed type. Thus, it is often the case thatusers directly view the fluorescent lamp that is mounted to an open-typeillumination apparatus. With this being the situation, it is importantto ensure the unevenness in the fluorescent lamp brightness due to thegap Ga is not noticeable.

To this end, the inventors of the present invention manufactured arctubes each having a different size of gap Ga between adjacent turns ofthe glass tube 112. Then, fluorescent lamps having those arc tubes weremounted to illumination apparatuses of a downwardly open type. Theillumination apparatuses then turned on to measure unevenness of thebrightness.

The glass tube constituting each arc tube body of the fluorescent lampsused in the measurement was 9.0 [mm] in outside diameter, 7.4 [mm] ininside diameter, and 700 [mm] in length. The glass tube was spirallywound, so that the gap between adjacent turns fell within a range of0.05 to 1.2 times the outside diameter of the glass tube. To be morespecific, there were glass tubes with a gap of 24 different sizes withinthe above-specified range in increments of 0.05 (i.e. 0.05, 0.1, 0.15,0.2 . . . 1.15, 1.2).

The evaluations of unevenness of the brightness were made in thefollowing manner. First, the fluorescent lamps were installed inceiling-mounted illumination apparatuses. The ceiling was at the heightof 3 [mm] from the floor and each illumination apparatus had the heightof 25 [mm]. The state of illumination of each lamp was subjectivelyobserved by 20 evaluators (ranging from 150 [mm] to 180 [mm] tall) whilewalking around on the floor. Unevenness of the brightness was rated on ascale of the following four levels: Not Noticeable; Not Very Noticeable;Rather Noticeable; and Noticeable.

FIG. 12 shows a table of the evaluations by the evaluators.

As apparent from the table, unevenness of the brightness was not verynoticeable when the gap Ga between adjacent turns of the glass tube wasequal to or less than 0.4 times the outside diameter of the glass tube.Furthermore, unevenness of the brightness was not very noticeable whenthe gap Ga was equal to or less than 0.3 times the outside diameter ofthe glass tube.

On the other hand, when the gap Ga was larger than 0.4 times the outsidediameter of the glass tube, unevenness of the brightness was rathernoticeable. When the gap Ga was larger than the 0.7 times, theunevenness was obviously noticeable.

Based on the above evaluations, when the gap Ga is equal to or less than0.4 times the glass tube 112, unevenness of the brightness is kept to anegligible level. Furthermore, by restricting the gap Ga to such a sizeis advantageous in terms of downsizing the arc tube 100.

Note that when the gap Ga of is equal to or less than 0.5 [mm], thepossibility increases that adjacent turns of the glass tube 112 comeinto contact upon compressing and deforming the intermediate product.

VI. Supplemental Notes

(1) Temperature of Intermediate Product

In the above description, the target temperature of the intermediateproduct 540 at the time of the compressing step is 600 [° C.](hereinafter, the target temperature may also be referred to as“compressing temperature”). Yet, the compressing temperature may be anytemperature that is equal to or higher than the temperature at which theglass tube 112 becomes deformable without cracking (550 [° C.],according to testing) and yet lower than the softening point of theglass tube 112 (675 [° C.]).

When the compressing temperature becomes equal to or higher than thesoftening point of the glass tube 112, the glass tube 112 is softenedand no longer capable of maintaining its shape. As a result, the glasstube 112 is distorted thereby causing the problems as follows. Forexample, the glass tube 112 may no longer be substantially circular inits cross-section, adjacent turns of the glass tube 112 may come intocontact, or the glass tube 112 is tapered at the end and thus prolongedin the axial direction.

In practice, it is preferable to determine the compressing temperaturein consideration of variations in the glass tube material as well as inthe temperature of the intermediate product 540. Thus, the compressingtemperature may be higher by 40 [° C.° C.] than the temperature at whichthe glass tube becomes deformable, and lower by 40° C. than thesoftening temperature. Within this temperature range (590 [° C.] to 635[° C.]), the intermediate product becomes easily deformable, and thecompressing temperature is easily controlled and managed.

(2) Shape of Conical Intermediate Product

The intermediate product 540 described above is substantially in aconical shape of which generating line and axis form angle α (see FIG.6A) that is 60°. Yet, the angle a may be any angle within a range of 45°to 70°.

When the angle a is larger than 70°, the height H of each of the grooves595 and 596 of the molding jig 590 becomes smaller than 0.5 [mm]. Thisconsiderably reduces the manufacturing yield of conforming items in theprocess of winding straight glass tubes into conical intermediateproducts. Furthermore, when the angle α is smaller than 45°, themanufacturing yield of conforming items is reduced in the process ofdeforming conical intermediate products into arc tube bodies having aflat spiral shape.

When the angle α is 60°, the manufacturing yield is 90 [%] or higher inthe process of winding straight glass tubes to spirally wound arc tubebodies. This manufacturing yield is the highest among the cases wherethe angle α was any other angle described above.

As described above, the height H of each of the grooves 595 and 596 isdetermined to be 0.56 times the outside diameter of the glass tube 510.The reason for determining the height H to such a size is to manufacturethe arc tube body 110 having the gap Ga of 1 [mm]. As described above,it is desirable that the gap Ga between adjacent turns of the glass tube112 to 1 [mm] to avoid uneven brightness. Another reason is to cause theangle β, which is an angle formed between the axis B and the generatingline B1 of the imaginary conical surface of the molding jig 590, to be60°. This angle is desirable for the above-identified reason.

Modifications

Up to this point, the present invention has been described by way of theabove embodiment. It should be naturally appreciated, however, that thepresent invention is in no way limited to the specific embodimentdescribed above. Various modifications including the following may bemade.

I. Pressurizing Intermediate Product

A. Pressuring Method

According to the above embodiment, pressure (i.e. load) is applied tothe intermediate product 540 by exploiting the weight of the movableplate 584. Yet, pressure may be applied by descending the movable plateunder displacement control.

Hereinafter, a description is given to an example in which the movableplate is mechanically descended to deform the intermediate product. Notethat the intermediate product and the compressing jig used in thisexample are identical to the ones described in the above embodiment.

First, the intermediate product is placed between the fixed plate andthe movable plate. Here, the movable plate is attached to a pressuredevice, in parallel relation to the fixed plate. The pressure device iscapable of controlling movement of the movable plate, for example, to aconstant speed.

Next, at least the wound portions of the intermediate product areheated. The heating is controlled so as to raise the temperature of thewound portions to the range of 620 [° C.]±10 [° C.].

When the temperature of the wound portions of the intermediate productreaches 620 [° C.] at its outer surface, the pressure device starts todescend the movable plate at a constant speed of about 4.0 [mm/sec].With this operation, the conical intermediate product is deformed into aflat arc tube body.

According to the above pressurizing method, the intermediate product iscompressed and deformed without causing distortion or clack in the glasstube. The percentage of obtaining conforming items according to theabove pressuring method is 97 [%] of the total number of intermediateproducts processed.

In the above testing using the mechanical pressurizing method, themovable plate is descended at 4.0 [mm/sec]. It has been confirmed thatthe descending speed may be within range of 3 [mm/min] to 5 [mm/min] tomanufacture arc tube bodies at high percentage of conforming items.

In the above example, the movable plate is descended under displacementcontrol. Yet, the flattening of the intermediate product may be carriedout by descending the movable plate under pressure control.Specifically, the wound portions of the intermediate product are heatedto a temperature equal to or higher than a point at which the glass tubebecomes deformable and yet lower than the softening point of the glasstube. The heated intermediate product is then compressed from thedirection of its imaginary axis. In this manner, the arc tube body isobtained in a desirable shape that is free from distortion of the glasstube.

In the above testing using the mechanical pressurizing method, thecompressing temperature is adjusted to 620 [° C.], which is differentfrom that of the above embodiment. Yet, the resulting arc tube body hasa desirable shape in cross section of the glass tube, similarly to theone obtained according to the above embodiment.

B. Pressurizing Direction

In the above embodiment and modification, the intermediate product 540is sandwiched between a pair of upper and lower plates (582 and 584),and the upper plate (584) is descended to compress the intermediateproduct 540. As an alternative, the upper plate may be made immovableand the lower plate may be ascended to sufficiently apply pressure tothe intermediate product. As another alternative, both the upper andlower plates may be inwardly moved to compress the intermediate product.

C. Additional Note

The inventors of the present invention have found the following, basedon observation of the intermediate product 540 being compressed anddeformed in the compressing step. When the weight of the movable plate584 is applied to the intermediate product 540, the deformation takesplace first at portions closer to the fixed plate 582 and then graduallyat portions closer and closer to the movable plate 584. The portion ofthe intermediate product in engagement against the movable plate 584 isdeformed at the latest.

It is considered that the intermediate product 540 deforms first atportions closer to the fixed plate 582 due to the following reason. Theturns of the wound portions 548 and 550 are lager in diameter atportions closer to the fixed plate 582 than at portions closer to themovable plate 584. Considering the wound portions of the intermediateproduct 540 as a spring, the spring constant is smaller at thelager-diameter portions. Thus, the wound portions are more easilydeformable at the portions closer to the fixed plate 582.

Based on the above observation, it is assumed that the by compressingthe innermost turn among the turns of the intermediate product, theouter turns are deformed before the innermost turn. This leads toeliminate the need to use, as in the above embodiment, the movable plate584 that is approximately as large as the fixed plate 582.Alternatively, the fixed plate may be sufficient if the size is largeenough to compress the innermost turn of the glass tube.

Note that in the above embodiment, the upper plate (equivalent to one ofthe pair of members of the present invention) is used as a movable body.Yet, the movable body is not limited to a plate or plate-like member.Alternatively, the movable body may be a cylindrical member (equivalentto one of the pair of members of the present invention) large enough tocompress the innermost turn of the glass member.

II. Compressing Jig

According to the above embodiment, the compressing jig 580 deforms thewound portions 548 and 550 of the intermediate product 540 by descendingthe movable plate 584. The intermediate product 540 may vary to someextent in its outer diameter and height of the overall shape as well asin the size of the gap between adjacent turns of the glass tube. Due tothe variations, it is difficult to apply uniform compressing forcethroughout the wound portions 548 and 550. As a result, the arc tubebody 110 obtained by deforming the intermediate product 540 may not havea uniform gap Ga between each two adjacent turns of the glass tube 112.

FIG. 13 is a plan view of a fixed plate of the compressing jig forforming an arc tube body having a substantially uniform gap between anytwo adjacent turns.

As shown in the figure, a fixed plate 682 is provided with holding pins683 for holding the turns of the wound portions 548 and 550 at equalradial intervals. Similarly to the above embodiment, the fixed plate 682is provided with stoppers 689 along the periphery thereof (in this case,the fixed plate is rectangular, so that the stoppers 689 are arrangedeach at one of the four corners). Also similarly to the aboveembodiment, guide pins 686 for guiding the movable plate are provided onthe fixed plate at positions surrounding the intermediate product 540.Naturally, the movable plate is provided with through holescorrespondingly to the holding pins 683 (not illustrated).

As shown in FIG. 13, a total of 16 holding pins 683 are provided alongtwo directions, one connecting ends 544 and 546 of the intermediateproduct, and the other perpendicular to the connecting direction. Yet,the number and positions of the holding pins 683 are not limited to thisspecific example shown in FIG. 13.

A greater number of holding pins more reliably keep adjacent turns ofthe wound portions 548 and 550 in equally spaced relation. With thenumber of pins, however, the time and trouble increase for placing theintermediate product on the fixed plate 682. Thus, it is preferable todetermine the number and positions of the holding pins 683 inconsideration of the above in balance.

III. Arc Tube Shape

A. EXAMPLE 1

According to the above embodiment, the arc tube 100 is in adouble-spiral shape that is formed by winding the glass tube 112 aroundthe imaginary axis A from the middle portion 120 all the way to therespective ends 114 and 116. Each turn of the spiral is diametricallylarger and larger as the turn is closer to the respective ends. Yet, thearc tube 100 may be in a single spiral shape that is formed by windingthe glass tube at least partially. Specifically, the arc tube may beformed by winding the glass tube from the middle portion at leastpartway to one end.

In order to form an arc tube body having a flat and single-spiral shape,an intermediate product having a conical and single-spiral shape needsto be manufactured first. In this case, the molding jig 590 shown inFIG. 7 is additionally provided with a through hole that coincides withthe axis B. The glass tube is made to run through the through hole fromone end to the middle portion. The end of the glass tube is fixed to themolding jig. Then, similarly to the above embodiment, the glass tube iswound around the imaginary surface of the molding jig from the middleportion toward the other end.

In order to flatten the wound portion of the thus obtained intermediateproduct, the fixed plate 582 shown in FIG. 10 is additionally providedwith a through hole for the non-wound portion of the intermediateproduct to pass through in the direction of the imaginary axis. With thenon-wound portion inserted trough the through hole, the movable plate isdescended in a similar manner to the above embodiment. As a result, anarc tube body having a flat and single-spiral shape is obtained.

B. EXAMPLE 2

According to the above embodiment, the arc tube body 110 has two woundportions that are formed by winding the two portions 510 b and 510 c ofthe glass tube 510 throughout their lengths. Yet, it is applicable thatonly part of the two portions 510 b and 510 c are wound around. Anexample of such an arc tube body may have a bend connecting a spirallywound potion and a non-wound portion extending to the end of the glasstube, in the direction of the imaginary axis A.

C. EXAMPLE 3

According to the above embodiment, the intermediate product 540 haswound portions with a substantially constant pitch both axially anddiametrically of the molding jig. Yet, the winding pitch does not haveto be constant.

FIG. 14A shows a modified shape of the intermediate product. FIG. 14Bshows a modified shape of the arc tube body.

As shown in FIG. 14A, an intermediate product 710 is in a conical spiralshape that are wound from a middle portion 712 toward ends 714 and 716thereof. The winding pitch of wound portions 718 and 720 is graduallylarger both axially and diametrically.

FIG. 14B shows an arc tube body 730 manufactured by compressing theabove-described intermediate product 710 from the direction of theimaginary axis. As shown in the figure, a gap between adjacent turns ofwound portions 738 and 740 is larger at a position of the glass tubefarther away from a middle portion 732 of the arc tube body 730 and thuscloser to respective ends 734 and 736 of the arc tube body 730. That isto say, the pitch of the winding is not constant and gradually larger inthe outwardly radial direction.

D. EXAMPLE 4

According to the above embodiment, the wound portions 548 and 550 of theintermediate product 540 are flattened throughout their lengths. Yet, itis applicable that only part of the wound potions is flattened.

FIG. 15 shows an arc tube body 750 formed by flattening part of thewound portions of the intermediate product 540 described in the aboveembodiment. As shown in the figure, the wound portions 548 and 550 onlyat part that is located within the lower half of the overall height ofthe intermediate product 540. In order to flatten only part of the woundportions, the movable plate 584 of the compressing jig 580 is providedwith a though hole that is diametrically larger than the through hole587, so as to accommodate therein the upper half of the wound portions548 and 550.

E. EXAMPLE 5

According to the above embodiment, the wound portions 548 and 550 spiralaround the conical surface of a circular cone. Thus, when seen from theaxial direction of the cone, the wound portions 548 and 550 aresubstantially circular around the imaginary axis A. Yet, it isapplicable that the wound portions may be wound around the imaginaryaxis A to form a polygonal shape when seen from the axial direction. Aglass tube may be wound to define a polygonal shape by using a moldingjig of a pyramidal cone having multiple triangular lateral faces.

IV. Glass Tube Material

According to the above embodiment, the glass tube is made of lead-freeglass. Yet, the material for the glass tube is not limited to lead-freeglass. Alternatively, hard glass, lead glass, and soda glass may beused. Naturally, each glass material (ingredient of glass) has its ownsoftening point. Thus, it is necessary to adjust the target temperatureat the time of heating the intermediate product in the compressing step.

According to the above embodiment, in addition, the outside diameter ofthe glass tube is 9.0 [mm]. It is preferable that the outside diameterof the glass tube falls within the range of 5 [mm] to 17 [mm] for thefollowing reasons. When the glass tube is smaller than 5 [mm] in outsidediameter, it is difficult to manufacture the arc tube, considering thesize of coli electrodes that need to be sealed within the grass tube. Onthe other hand, when the glass tube is larger than 17 [mm] in outsidediameter, the resulting arc tube becomes relatively large.

V. Bulge of Arc Tube

According to the above embodiment, the bulge 126 of the arc tube 100 isformed at the apex of the intermediate product 540 before deforming thewound portions 548 and 550. As an alternative, the bulge 126 may beformed after deforming the wound portions 548 and 550. To this end, theapex and its nearby portion of the flattened arc tube body are locallysoftened, followed by increasing the internal pressure of the arc tubebody. This modification eliminates the need to provide the through hole587 in the movable plate 584 of the compressing jig 580.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless such changes and modifications depart fromthe scope of the present invention, they should be construed as beingincluded therein.

1. A manufacturing method of an arc tube, comprising: a winding step ofspirally winding a glass tube around an imaginary conical surface; and adeforming step of heating a spiraled portion of the glass tube to atemperature equal to or higher than a point at which the glass tube isdeformable and lower than a softening point of the glass tube, anddeforming the spiral so that the spiraled portion has a tube axissubstantially within one plane.
 2. The manufacturing method according toclaim 1, wherein the glass tube before the winding has two portions tobe wound and a middle portion between the two portions, and in thewinding step, the two portions are wound with the middle portion fixedin place at a position corresponding to an apex of the imaginary conicalsurface.
 3. The manufacturing method according to claim 1, wherein whenseen from an axis of the imaginary conical surface, a minimum gapbetween adjacent turns of the spiraled portion before the deforming isnot smaller than 0.5 mm and not greater than 0.4 times an outsidediameter D1 of the glass tube.
 4. The manufacturing method according toclaim 2, wherein when seen from an axis of the imaginary conicalsurface, a minimum gap between adjacent turns of the spiraled portionbefore the deforming is not smaller than 0.5 mm and not greater than 0.4times an outside diameter D1 of the glass tube.
 5. The manufacturingmethod according to claim 1, wherein an angle formed between agenerating line and an axis of the imaginary conical surface is within arange to 45° and 70°.
 6. The manufacturing method according to claim 4,wherein an angle formed between a generating line and an axis of theimaginary conical surface is within a range to 45° and 70°.
 7. Themanufacturing method according to claim 1, wherein in the deformingstep, the spirally wound portion of the glass tube is placed between apair of members having parallel opposing surfaces, and compressed by thepair of members.
 8. The manufacturing method according to claim 6,wherein in the deforming step, the turns an angle formed between agenerating line and an axis of the imaginary conical surface is within arange to 45° and 70°.
 9. The manufacturing method according to claim 2,wherein in the deforming step, the spirally wound portion of the glasstube is placed between a pair of upper and lower plates so as to bringthe middle portion into engagement against the upper plate, the pair ofplates having opposing surfaces each of which is substantiallyhorizontal, and the spiral is compressed under a weight of the upperplate.
 10. The manufacturing method according to claim 6, wherein in thedeforming step, the spirally wound portion of the glass tube is placedbetween a pair of upper and lower plates so as to bring the middleportion into engagement against the upper plate, the pair of plateshaving opposing surfaces each of which is substantially horizontal, andthe spiral is compressed under a weight of the upper plate.
 11. Themanufacturing method according to claim 9, wherein the upper plate has acavity at a position corresponding to an apex of the middle portion forallowing the apex to at least partially enter the upper plate.
 12. Anarc tube comprising a glass tube having at least one portion spirallywound around an imaginary axis, wherein the spiraled portion of theglass tube has a tube axis substantially within one plane perpendicularto the imaginary axis, and a minimum gap between adjacent turns of thespiraled portion is not less than 0.5 mm and not greater than 0.4 timesan outside diameter D1 of the glass tube in mm.
 13. The arc tubeaccording to claim 12, wherein the glass tube has two portions spirallywound around the imaginary axis extending through to respective ends ofthe glass tube from a middle portion of the glass tube.
 14. The arc tubeaccording to claim 13, wherein an outside diameter of the glass tube isnot less than 5 mm and not greater than 17 mm.
 15. A low-pressuremercury discharge lamp comprising the arc tube as defined in claim 12.16. An illumination apparatus comprising the low pressure discharge tubeas defined in claim 15.